As this type of apparatus, for example, there is proposed an optical sensor having a light emitting element and a light receiving element, which are mounted on an insulated substrate, the optical sensor being provided with a cap for the light receiving element, the cap covering only the light receiving element and having an entrance window and a light polarizing surface (refer to Patent Literature 1). Alternatively, there is proposed a biological information monitoring system provided with a laser blood flowmeter configured to measure a blood flow on the basis of scattered light caused by scattering of laser light in a biological tissue (refer to Patent Literature 2).